Alzheimer's disease is a progressive disease known generally as senile dementia. Broadly speaking the disease falls into two categories, namely late onset and early onset. Late onset, which occurs in old age (65+ years), may be caused by the natural atrophy of the brain occurring at a faster rate and to a more severe degree than normal. Early onset Alzheimer's disease is much more infrequent but shows a pathologically identical dementia with diffuse brain atrophy which develops well before the senile period, i.e., between the ages of 35 and 60 years. There is evidence that one form of this type of Alzheimer's disease shows a tendency to run in families and is therefore known as familial Alzheimer's disease (FAD).
In both types of Alzheimer's disease the pathology is the same but the abnormalities tend to be more severe and more widespread in cases beginning at an earlier age. The disease is characterized by two types of lesions in the brain, these are senile plaques and neurofibrillary tangles.
Senile plaques are areas of disorganized neuropil up to 150 .mu.m across with extracellular amyloid deposits at the center. Neurofibrillary tangles are intracellular deposits of amyloid protein consisting of two filaments twisted about each other in pairs.
The major protein subunit, .beta.-amyloid protein, of the amyloid filaments of the senile plaque is a highly aggregating small polypeptide of approximate relative molecular mass 4,500. This protein is a cleavage product of a much larger precursor protein called amyloid precursor protein (APP).
At present there is no known effective therapy for the various forms of Alzheimer's disease (AD). However, there are several other forms of dementia for which treatment is available and which give rise to progressive intellectual deterioration closely resembling the dementia associated with Alzheimer's disease. A diagnostic test for AD would therefore provide a valuable tool in the diagnosis and treatment of these other conditions, by way of being able to exclude Alzheimer's disease. It will also be of value when a suitable therapy does become available.
Also important is the development of experimental models of Alzheimer's disease that can be used to define further the underlying biochemical events involved in AD pathogenesis. Such models could presumably be employed, in one application, to screen for agents that alter the degenerative course of Alzheimer's disease. For example, a model system of Alzheimer's disease could be used to screen for environmental factors that induce or accelerate the pathogenesis of AD. In contradistinction, an experimental model could be used to screen for agents that inhibit, prevent, or reverse the progression of AD. Presumably, such models could be employed to develop pharmaceuticals that are effective in preventing, arresting, or reversing AD.